1. Field of the Invention
The present invention relates to a combined power generation plant.
2. Description of Related Art
Conventionally, a combined power generation plant in which a gas turbine, exhaust gas boiler (waste heat recovery boiler), and steam turbine are combined has been used as a thermal power generation plant.
FIG. 6 shows one example of a well-known multi-shaft reheat type combined power generation plant. This plant includes gas turbines 601, exhaust gas boilers 602, a steam turbine 603, and generators. Although three gas turbines 601 and three exhaust gas boilers are provided, only one each is shown, and No. 2 and No. 3 gas turbines and exhaust gas boilers are omitted in the figure.
The gas turbine 601 takes in air from the compression side, feeds burned fuel to the expansion side, and sends exhaust gas to the exhaust gas boiler 602. This gas turbine 601 generates electric power.
The exhaust gas boiler 602 uses a triple pressure reheat type construction having low-, intermediate-, and high-pressure waste heat recovery sections. Specifically, this exhaust gas boiler includes a low-pressure economizer (LP ECO) 604, a low-pressure evaporator (LP EVA) 605, a high- and intermediate-pressure economizer (IP ECO & HP ECO1) 606, an intermediate-pressure evaporator (IP EVA) 607, a low-pressure superheater (LP SH) 608, an intermediate-pressure superheater (IP SP) 609, a high-pressure economizer (HP ECO2) 610, a high-pressure evaporator (HP EVA) 611, a high-pressure superheater (HP SH) 612, and a reheater (IP RH) 613. The operations of these pieces of the aforesaid equipment are the same as those of the publicly known equipment. Also, an NOx removal system 614 is provided at the subsequent stage of the high-pressure evaporator 611.
Further, BFP stands for a boiler feed water pump. CP stands for a condensed water pump. GC stands for a grand condenser heater.
FIG. 7 shows a sequence for waste heat recovery in the aforesaid waste heat recovery equipment. In FIG. 7, the abbreviation indicated by alphabets corresponds to each of the aforesaid waste heat recovery equipment.
The exhaust gas boiler 602 recovers waste heat from the exhaust gas introduced from the inlet by using the aforesaid equipment, and feeds the generated steam to the steam turbine 603 to generate electric power. The exhaust gas is finally discharged through a stack. In the figure, the solid lines indicate the flow of feed water, and the dotted lines indicate the flow of steam.
To the steam turbine 603, steam is sent from not only the shown No. 1 exhaust gas boiler 602 but also No. 2 and No. 3 exhaust gas boilers. Also, water from a condenser and make-up water are sent to these exhaust boilers through lines.
A power generation plant having the same configuration as that of the example shown in FIG. 6, which uses 1350.degree. C.-class gas turbines, really exists. Its total output at an atmospheric temperature of 22.degree. C. is 670 MW, and the plant efficiency (generating end) is 48.7%.
The reason why the exhaust gas boiler 602 is of a triple pressure reheat type having low-, intermediate-, and high-pressure waste heat recovery sections is that as seen from FIG. 7, waste heat cannot be recovered well at low temperatures by the high-pressure waste heat recovery section (HP ECO, HP EVA, HP SH) only.
From an idealistic viewpoint, high-pressure waste heat recovery should mainly be carried out. In this case, however, the temperature of exhaust gas must be increased. For this purpose, a method has been tried in which some LNG gas is supplied in the exhaust gas boiler and burned. However, this method is impractical for the reason that it is difficult to properly control incomplete combustion and combustion condition and other reasons. For example, the piping material is designed so as to match the assumed temperature.